The mechanism of repair of UV-induced DNA damage is studied. Human lymphoblastoid cell lines, including normal and xeroderma pigmentosum, which differ in their ability to repair UV-induced DNA damage, are exposed to different doses of 254 nm UV radiation. The cells are allowed to repair the damage in the presence of radioactive nucleotides. Chromatin from these cells is isolated and the location of the repair site identified with the aid of specific nucleases. DNA synthesis in the presence of hydroxyurea results in incorporation of counts which are particularly sensitive to DNase digestion. The major aim of the project is to identify repair sites and to explore the possibility that differences between various cells in UV sensitivity reflect some differences in the repair sites.